1. Field of the Invention
The present invention relates to a fluorescent material for fabricating high brightness white light emitting diodes. More particularly, the present invention relates to a fluorescent material having a formula (YxMyCez)Al5O12, in combination with purple-blue light or blue light emitting diodes, for generating a high brightness white light emitting diode device.
2. Description of the Prior Art
In 1996, a Japanese company, Nichia Kagaku Kogyo Kabushiki Kaisha (Tokushima), which is also known as xe2x80x9cNichia Chemicalxe2x80x9d on the market, disclosed a method for generating white light by using a blue light emitting diode (LED) that emits blue light absorbed by a fluorescent material to emit yellowish light. The yellowish light is diffused and mixed with blue light to eventually generate high brightness white light. This new technique has ushered a new era of white LED illumination and is believed that will soon replace the conventional fluorescent lamps in the near future. In Taiwan Patent No. 1,561,77I and also in U.S. Pat. No. 5,998,925, assigned to Nichia Kagaku Kogyo Kabushiki Kaisha, disclose a yellow light YAG:Ce fluorescent powder, which has a general formula (Yl-p-q-rGdpCeq Smr)3(All-sGas)5O12, where 0xe2x89xa6pxe2x89xa60.8, 0.003xe2x89xa6qxe2x89xa60.2, 0.003xe2x89xa6rxe2x89xa60.08, 0xe2x89xa6sxe2x89xa61. The blue LED irradiates a fluorescent material to general yellowish light that is complementary to the blue light. The blue light and the yellowish light are then mixed using lens theory to general visible light that is white color to human eye. This can be further explained by using a so-called C.I.E. chromaticity diagram. When wavelength of light emitted by the semiconductor light-emitting component corresponds to a point on a straight line connecting point A (blue light) and point B (yellowish light) in a chromaticity diagram, light of white color can be emitted.
It is known that the white light is mixed light of different colors. The white light, which is sensed by human eye as white color, at least includes two or more colors of light having different wavelengths. For example, when human eye is stimulated, at the same time, by the Red, Green, and Blue colors of light, or by blue light and yellowish light, a white color is sensed. Accordingly, there have been three major approaches to the formation of white light for now. The first is using R/G/B LEDs. By controlling the current passing the LED to generate white light. The second is using yellow/blue LEDs to generate white light. These two prior art methods has a common drawback in that when quality of one of the plural LEDs deteriorates, an accurate white light is no longer obtained. Furthermore, using plural LEDs is costly. Another known approach is using InGaN LED, which generates blue light that. can be absorbed by fluorescent dye or powders to emit yellowish light, that is mixed with blue light to produce white light. This newly developed has no disadvantage of the former two prior art methods as described above. Besides, such LED has a simpler driving circuit and can be made by simple manufacturing process. Further, such InGaN LED has low power consumption and cost. As a result, the third approach (InGaN LED) is widely used for various white LED applications. However, so far, since most commercial InGaN type blue LED is made by using metal organic chemical vapor deposition (MOCVD), only blue LED with fixed wavelength can be obtained. There has been a strong need for providing a series of yellow light fluorescent powders capable of modulating emitted blue light wavelengths in a range of from 430 nm to 490 nm.
As disclosed in Taiwan Patent No. 1,561,77I and also in U.S. Pat. No. 5,998,925, the light color modulating method used by Nichia Chemical includes the step of adding a hetero ion. For example, when the general formula (Yl-p-rGdpCeqSmr)3 (All-s,Gas)5O12 is (Yl-q Ceq)3Al5O12, 546 nm yellowish light is obtained. But, after adding Gd into this formula, the resulting (Yl-p-qGdpCeq)3Al5Ol2 formula can red shift the main wavelength to 556 nm.
The main objective of the invention is to provide an alternative approach to produce red shift. This is achieved by adjusting magnitude of crystal field of the host of a fluorescent material. The electron configuration of three valance Ce is [Xe]4f1, wherein the 4f orbital is split by spin-orbitalal coupling into 2F5/2 and 2F7/2, and wherein the 5d orbital is split due to crystal field interactions. As the crystal field increases, the level of the split 5d orbital also increases. This results in the reduced energy difference between the lowest energy level of the 5d orbital and the 4f orbital of the excited Ce3a+, thereby generating red shift of light.
In accordance with the present invention, a composition of a fluorescent material having a formula (YxMyCez)Al5O12 is provided, wherein x+y=3, x, yxe2x89xa00, 0 less than z less than 0.5, M is selected from the group consisting of Tb, Lu, and Yb, wherein the (YxMy)Al5O12 is a host thereof, and Ce is an activator thereof, and wherein by adjusting the metal component of the (YxMy)Al5O12 host of the fluorescent material, a crystal field thereof can be modulated, thereby changing wavelength of light emitted from the fluorescent material.
Other objects, advantages and novel features of the invention will become more clearly and readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.